1. Field of the Invention
The present invention relates to a frequency conversion unit, more particularly to a frequency conversion unit which contains a mixer which mixes an oscillation output from a local oscillator with an intermediate frequency input and thereby produces a frequency conversion output signal. The frequency conversion unit to which the present invention is concerned is mainly intended to be used in a communication system.
2. Description of the Prior Art
One of the functions required of a frequency conversion unit is low local frequency leakage. The local oscillation output for the frequency conversion should be used only for achieving the mixing of the frequencies. Therefore, the local oscillation output should not leak into the resultant frequency conversion output signal. That is, it is desirable to reduce the local leakage level to a very low level, zero if possible.
A typical frequency conversion unit, as described hereinafter, has a shortcoming of requiring an additional, special band elimination filter, a band-pass filter, and isolators for suppressing the local leakage. The isolators also absorb spurious waves and simultaneously match impedance.
To deal with the above shortcoming, U.S. Pat. No. 4,249,263 proposed a mixer circuit able to suppress the local leakage without employing such a special band elimination filter, a band-pass filter, or isolators. This mixer circuit, also described hereinafter, is not so superior in view of the poor impedance matching characteristics caused between the local oscillator and a circuit cooperating therewith.